In general, as a wire for a high-temperature superconducting cable, a Bi (bismuth)-based silver-sheathed superconducting wire and an Y (yttrium)-based thin-film superconducting wire are known. The problem with the Bi-based silver-sheathed superconducting wire is that the critical current density is sharply decreased when an external magnetic field is applied thereto. Patent Document 1 describes that, in a superconducting cable using the Bi-based silver-sheathed superconducting wire, a plurality of tape-shaped Bi-based silver-sheathed superconducting wires having the same section size are wrapped around a cylindrical former in multiple layers in such a way that no circumferential clearance is left between the adjacent superconducting wires in all layers, whereby a magnetic field component applied to a larger-width face of the superconducting wire in a vertical direction is reduced, and thereby reducing degradation of critical current and reducing the alternating-current loss.
On the other hand, the Y-based thin-film superconducting wire is expected to be applied to an alternating-current power apparatus such as a superconducting cable, because it is able to maintain a high current density even in a strong magnetic field. Furthermore, as a result of the Y-based thin-film superconducting wire being formed by evaporating a YBCO thin film onto a metal substrate, it is a thin film and has a high current density, holding greater promise to reduce a loss (alternating-current loss) produced in alternating-current conditions than the Bi-based silver-sheathed superconducting wire on the elemental wire level.
Because of the extreme thinness of its superconducting material, the Y-based thin-film superconducting wire is known to produce almost no alternating-current loss due to a magnetic field component parallel to a larger-width face of a tape wire. Therefore, an ideal superconducting cable using the Y-based thin-film superconducting wire has a structure in which the Y-based thin-film superconducting wires are disposed with no clearance between them, and, in that case, a self-magnetic field exists only in a conductor circumferential component, making it possible to dramatically reduce the alternating-current loss. Ultimately, as shown in FIG. 10, it is preferably circular in cross section (cylindrical). However, with a superconducting elemental wire having an intermediate layer and a superconducting layer formed on a cylindrical base, by a method described in Patent Document 2 in which it is manufactured by disposing a target for each layer on both upper and lower sides of the wire, it is difficult to align a crystal axis direction even when the superconducting layer is formed so as to be circular in cross section as shown in FIG. 10. It has been found out that, to bring it close to this shape, it is simply necessary to shape it into a polygon having as many vertices as possible by making the Y-based thin-film superconducting wire having a finite width thinner.    [Patent Document 1]            JP-A-9-190727            [Patent Document 2]            JP-A-2000-106043        